In exhaust gases which are emitted from internal combustion engines, such as diesel engines, particulates are included. In the particulates, substances which are harmful to human bodies are included, and it has become an environmental assignment to remove them.
For removing the particulates, filter catalysts have been used. A conventional filter catalyst has a construction in which a catalytic layer comprising a loading layer composed of alumina, and the like, and a catalytic metal loaded on the loading layer is formed on a catalyst-support substrate composed of porous ceramic having chained pores. And, when exhaust gases pass the ventilation holes formed by the chained pores of the catalyst-support substrate, it captures and decomposes the particulates. At this moment, if the ventilation holes are not formed in the filter catalyst to the extent that the exhaust gases pass, the captured particulates deposit to close the ventilation holes so that the pressure loss increases when the exhaust gases pass. When the opening diameters of the ventilation holes are enlarged in order to reduce the pressure loss, it has become impossible to capture the particulates.
The conventional filter catalyst has been produced by forming a loading layer by preparing a coating slurry, such as alumina, coating this coating slurry onto a catalyst-support substrate, and drying-calcining it (coating); and thereafter loading a catalytic ingredient thereon. For example, it is described in Japanese Unexamined Patent Publication (KOKAI) No. 9-173,866 or Japanese Unexamined Patent Publication (KOKAI) No. 9-220,423.
In Japanese Unexamined Patent Publication (KOKAI) No. 9-173,866, there is set forth a production process in which coating is carried out by immersing a catalyst-support substrate into an alumina slurry and taking it up therefrom, and thereafter suctioning the alumina slurry in excess with an air cleaner, or blowing it off with a compressed air.
In Japanese Unexamined Patent Publication (KOKAI) No. 9-220,423, there is set forth a production process in which coating is carried out by pouring a coating slurry into a catalyst-support substrate, whose axial direction is disposed vertically, through the top end, or pushing up the coating slurry through the bottom end, to coat the slurry, and suctioning the coating slurry in excess by decompressing the bottom-end side, or washing away the coating slurry by pressurizing the top-end side.
However, in the production methods set forth in the aforementioned respective publications, there have been such a problem that the produced catalytic layer narrows down the opening diameters of the ventilation holes or closes the pores of the catalyst-support substrate. To be more precise, the removing of the excessive coating slurry from the catalyst-support substrate with the coating slurry coated has been carried out by giving a large pressure difference to the both opposite ends of the catalyst-support substrate at once. In such a method, the particles constituting the coating slurry, such as alumina, come to be present un-uniformly within the pores of the catalyst-support substrate. And, since they are dried-calcined in this state, the produced catalytic layer comes to narrow down the opening diameters of the ventilation holes or close them.
Moreover, since the pores of the catalyst-support substrate are small, the ventilation holes of the obtained catalytic layer come to be closed, when the particle diameters of the particles constituting the coating slurry become large. Further, the coating slurry does not distribute inside the pores of the catalyst-support substrate so that the catalytic layer comes to be formed on the surface of the catalyst-support substrate, when the particle diameters of the particles constituting the coating slurry become large. Specifically, the catalytic layer is formed on the surface of the catalyst-support substrate, and this catalytic layer closes the pores of the catalyst-support substrate so that no sufficient ventilation property as filter catalyst comes to be secured. When no ventilation property comes to be secured, the pressure loss heightens so that loads come to be applied to engines.